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| Home > Publications database > Geochemical significance of heterocompounds in petroleum systems, offshore Norway |
| Dissertation / PhD Thesis/Book | PreJuSER-36648 |
2004
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/162
Report No.: Juel-4118
Abstract: This thesis presents the results of an organic geochemical investigation of 72 oils, nine source rock samples and two core samples from the main petroleum systems located offshore Norway. The main focus of the study was on the geochemical significance of nitrogen, sulphur and oxygen (NSO) compounds in relation to source facies and maturity, primary and secondary migration, and biodegradation. Samples comprising an artificial maturity sequence, obtained by hydrous pyrolysis of an immature source rock, were also used for an investigation of the effects of maturity and expulsion on polar compound composition. During the course of this study, the occurrence of xanthones in crude oils was detected and their gas chromatographic elution order was measured for the first time. The crude oils were initially classified into broad facies, maturity and biodegradation categories on the basis of their hydrocarbon geochemistry. The geochemical behaviour of three heterocompound classes, i.e. carbazoles, fluorenones and xanthones, was then investigated in detail, and the data were interpreted within the context of these important geochemical controls and processes. The two potential source rocks from the Haltenbanken area could be differentiated by the distribution of carbazoles and fluorenones, independent of maturity. The discrimination parameters include the relative concentrations of the compound groups, several ratios of alkylated carbazoles and fluorenones, and the distributions of benzocarbazoles and benzofluorenones. The absolute concentrations of these compound classes are strongly dependent on maturity, and this is convincingly demonstrated by the analysis of the natural source rock and the recovered rock extracts from the artificial maturation experiment. A maturity influence is observed for some ratios of alkylated carbazoles and fluorenones, and for the distribution ofthe benzo-isomers in the extracts. Whereas facies and maturity clearly control the distribution of these constituents in source rocks partitioning processes are thought to be the main control on the composition of these polar compounds in oils. The results of this study indicate different scenarios for the three compound classes in crude oils. During petroleum expulsion, most of the carbazoles generated are retained within the source rock due to their moderate polarity. There is, however, an isomeric fractionation during primary migration such that shielded carbazoles are enriched relative to non-shielded carbazoles in the expelled crude oils. The effects of primary migration are enhanced during secondary migration due to the interaction between carbazoles and the aqueous, organic and mineral phases of the carrier system. In comparison to carbazoles, fluorenones are generally more strongly retained by the source rock during expulsion due to their higher polarity. Low-polarity fluorenes which are released during expulsion are easily oxidised to the corresponding ketones during migration through interaction with water and/or mineral matter. This susceptibility to oxidation is due to the activation of the methylene bridge by the two adjacent benzene rings. An observation of relative enrichment of the non-shielded methylfluorenones with increasing petroleum secondary migration distance is attributed to the reduced susceptibility of 1-methylfluorenone to oxidation due to steric hindrance. In contrast to high concentrations observed for the carbazoles and fluorenones in the source rocks, the highly-polar xanthones were detected either in minor amounts or were absent. However, concentrations of xanthones similar to those of the other two compound classes were detected in the crude oils. This indicates that xanthones probably originate by geosynthesis from suitable aromatic precursors within carrier systems. Xanthones therefore show great promise as indicators of the degree of organicinorganic interaction in carrier systems. This assumption is supported by an increase in the concentration of xanthones in oils with increasing migration distance. The ratio of the shielded methylxanthones to the non-shielded isomers increases with increasing migration distance in the same way as for the carbazoles. This observation conforms to oil-water partition behaviour as determined by molecular dynamics calculations. Biodegradation of petroleum within the reservoir can also influence the occurrence and distribution of these polar constituents in oils. The non-shielded isomers of carbazoles and xanthones are preferentially biodegraded compared to the shielded isomers due to their higher affinity for water. In contrast, the concentration of partially shielded 1-methylfluorenone decreases with increasing degree of biodegradation compared to its non-shielded isomers. The reason for the decrease in relative abundance of the 1-methylfluorenone may be related to biological degradation of fluorenes, as biological degradation of fluorene results in the production of fluorenone as a stable intermediate. The oxidation of 1-methylfluorene to its corresponding ketone is sterically hindered by the methyl group adjacent to the methylene bridge.
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